Social networks are services, for example computer programs running on servers that allow users to share content, for example pictures, text, or audio/video. Typically, social networks only allow write access to the content to registered users. Access control is typically performed by said servers using credentials, for example username and password, in order to grant access to reading or writing of content in said social network.
Each user typically has its own user account on the social network, allowing to relate any content uploaded, using a specific account to the user that owns the account of the social network.
This means, that everyone who wants to participate in said social network needs to register first, for example to create an account on said server hosting said social network.